PURPOSE: To reduce an interval time required to replace molds by comparing which is greater between the amount of a first resin filled in a first mold and that of a second resin filled in a second mold.

CONSTITUTION: The amount of a first resin filled in a first mold 1 is compared with that of a second resin filled in a second mold 2 to find which is greater. If the mount of the first resin is found to be greater than that of the second resin, the molding is repeated again with the first mold 1 after the production with the first mold 1 is completed. As the result of disposing the resin measured for the sake of the first mold 1 into the same first mold 1, purging is no longer necessary. On the other hand, if the amount of the second resin is greater than that of the first resin, the resin measured for the sake of the first mold 1 can by disposed into the second mold 2, but with its additional amount to be supplemented. Accordingly, it is prevented that after the release of mold a molded product cannot be ejected with an ejector pin because of contraction void that would occur in the case where the resin in an amount less than necessary is thrown into the second mold 2.